Currently, filler levels in paper are limited, in part, by paper strength losses as filler levels increase. Minimum strength requirements prohibit the paper maker from adding more filler which is desirable because fillers are generally less expensive than the wood fiber that they replace. The ability to add more filler would allow the paper maker to reduce paper production costs by replacing the more expensive fiber. In order to achieve higher filler levels, the paper maker needs fillers, additives or processes which increase paper strength. Historically, the paper maker has used filler modification, wet end additives and fiber modifications to achieve higher strength levels in paper. Filler modifications have included changing filler type, changing filler particle size and surface treatments of fillers. Wet end additives have included synthetic and natural polymers such as polyacrylamides and starches. Fiber modifications have included changing fiber type and fiber processing.
In general, the paper industry is trending toward higher filler levels to reduce costs and as a result is continually looking for ways to improve paper strength. To achieve this, a new generation of strength aids is required for the paper industry.
In U.S. Pat. No. 3,709,780, there is disclosed paper products having improved strength properties by adding to the cellulose paper dispersion a chitin-based compound of a graft copolymer of 2-acrylamide-2-methylpropanesulfonic acid (AMPS) onto a chitosan substrate.
In U.S. Pat. No. 3,692,673, there is disclosed polymers of acrylamido sulfonic acids, and their salts, which are useful as flocculants or aqueous systems.
In U.S. Pat. No. 4,555,558, there is disclosed water soluble terpolymers of AMPS; N,N-dimethylacrylamide; and acrylonitrile. These terpolymers are reported to provide high temperature fluid loss additives and rheology stabilizers for high calcium-containing, brine clay drilling fluids.
In U.S. Pat. No. 3,926,718, there is disclosed the use of water-soluble block polymers having blocks derived from a water-soluble monomer and blocks derived from N-vinyl pyrrolidone. These polymers are reported to perform as drainage and retention aids for paper webs where the polymer is added to a pulp slurry.
In U.S. Pat. No. 5,075,401, there is disclosed a graft copolymer made by a free radical reaction mechanism. The copolymer uses polymeric units of acrylamide, acrylic acid and/or AMPS.